1. Field of the Invention
The present invention relates to an ink jet printing apparatus and method to form a uniform image.
2. Description of the Related Art
A printing apparatus of an ink jet printing system (hereinafter referred to as an ink jet printing apparatus) performs a printing operation by ejecting ink from a print head onto a print medium and can easily be upgraded to a higher resolution, compared with other printing systems. The ink jet printing apparatus also has advantages of high speed printing capability, low noise and low cost. As there are growing needs for color output in recent years, a printing apparatus capable of producing high-quality printed images matching silver salt pictures in quality has been developed.
The ink jet printing apparatus incorporates a print head having a plurality of print elements (electrothermal transducer or piezoelectric element) densely arrayed therein for higher printing speed. Also for a color printing capability, many printing apparatus are provided with a plurality of such print heads.
FIG. 1 shows a construction of main components of a general ink jet printing apparatus. In the figure, denoted 1101 are ink jet cartridges. Each of these has a combination of an ink tank containing one of four colors, black, cyan, magenta and yellow, and a print head 1102 corresponding to the ink.
FIG. 2 shows a group of the ejection openings for one color arrayed corresponding to the print elements of the print head 1102, as seen from a direction of arrow Z of FIG. 1. In the figure, denoted 1201 are ejecting openings that number d and are arranged at a density of D openings per inch (D dpi). Hereinafter, a constitution including a print element and an opening corresponding to that is referred to as a nozzle.
Referring again to FIG. 1, reference number 1103 represents a paper feed roller, which, together with an auxiliary roller 1104, holds a print medium P and rotates in the direction of arrow to feed the print medium P in the direction of arrow Y (subscan direction). Denoted 1105 are a pair of supply rollers that supply the print medium P. The paired supply rollers 1105, as with the rollers 1103 and 1104, hold the print medium P between them and rotate at a slightly lower speed than the paper feed roller 1103, thereby applying an adequate level of tension to the print medium.
Denoted 1106 is a carriage that supports the four ink jet cartridges 1101 and moves them as the cartridges perform a scan. The carriage 1106 stands by at a home position h shown with a dashed line when the printing operation is not performed or when a recovery operation on the print head 1102 is executed.
When a print start command is entered into the printing apparatus, the carriage 1106 standing by at the home position h moves in the X direction (main scan direction) and at the same time the print heads 1102 on the carriage eject inks at a predetermined frequency from the nozzles 1201, forming a band of image d/D inch wide on the print medium. After the first printing scan is finished and before the second printing scan starts, the paper feed roller 1103 rotates in the direction of arrow to feed the print medium a predetermined distance in the Y direction. These main printing scan and feeding operation are alternated repetitively to produce an image in a stepwise fashion.
Such an ink jet printing apparatus often employs a multi-pass printing method. The multi-pass printing method will be briefly explained below.
In the multi-pass printing, image data that can be printed in one main printing scan is thinned by a mask pattern before executing the main printing scan. Further, in the next printing scan, image data that is thinned by a mask pattern complementary to the already used mask pattern is printed. Between each printing scan, a feed operation is performed to feed the print medium a distance shorter than the print width of the head.
In the case of a 2-pass printing, for example, a mask pattern used in each main printing scan thins the image data by about 50%. The distance that the print medium is fed by the feed operation is one-half the print width. By repeating the above printing operation, dots arrayed on a line leading to the main scan direction are printed by two different nozzles. Thus, since the print data is divided into halves and distributed among the two different nozzles, even if individual nozzles have some ejecting variations, an image produced is smoother than that produced by a 1-pass printing that does not use the multi-pass printing. Although the 2-pass printing has been explained here, the image produced by the multi-pass printing can be made smoother by increasing the number of passes (division number). This, however, results in an increased number of main printing scans and feed operations and therefore an increased output time. To reduce the output time as much as possible, a bidirectional multi-pass printing has become a mainstream in recent years which ejects ink in both forward and backward directions.
When ink is ejected from the nozzles of the ink jet print head, fine sub droplets of ink may be ejected along with main droplets that are intended to form an image. In the following description, dots formed by the main droplets are called main dots and dots formed by sub droplets satellites. The above relation between the main droplet and the sub droplet holds in one ejection. The one ejection referred to here is an ejection performed in response to one electric signal. The sub droplet is characterized by a slower ejection speed and a smaller volume than those of the main droplet. It is noted, however, that the satellites are not always smaller in size than the main dots.
FIGS. 3A to 3D show landing positions on a print medium of a main dot and a satellite. In these figures, 1301 represents a main dot and 1302 a satellite. An arrow shown in an upper part of these figures indicates a direction in which a carriage moves during the ejection operation. An arrow shown in a lower part of the figures indicates a direction in which a droplet is ejected.
FIG. 3A shows dots formed when the direction of ejection is vertical to the print medium. Normally if the print head is not inclined, the ejection face of the print head is parallel to the print medium and the direction of ejection is therefore vertical. Generally the sub droplet is slower in ejection speed than the main droplet and therefore lands on the print medium lagging behind the main droplet. During ejection, the carriage is moving in the direction of arrow 1303 in the figure, so the carriage speed is added to the ejection speed of the droplet, with the result that the landing time difference results in a landing position difference in the main scan direction.
FIG. 3B illustrates dots formed when the direction of ejection includes a component of the carriage movement. If the ink droplet ejection direction has some inclination due to various factors, such as a nozzle material swelling or the ink to be ejected being pulled into the liquid chamber, the ejection face of the head is not parallel to the print medium, forming dots as shown in FIG. 3B. In that case, the velocity components of the main droplet and sub droplet are each given the component of arrow 1304. Thus, the distance between the main dot 1301 and the satellite 1302 in the main scan direction further increases.
FIG. 3C illustrates dots formed when the ejection direction has an inclination opposite to that of FIG. 3B and includes a component (arrow 1305) opposite to the direction of carriage movement. In this case, the velocity components of the main droplet and sub droplet are the ejection direction component 1305 subtracted from the carriage velocity component 1303. Thus, the distance between the main dot 1301 and the satellite 1302 is shorter than that of FIG. 3A. FIG. 3C shows the satellite contained in the main dot when they land.
FIG. 3D illustrates dots formed when the velocity component is the same as that of FIG. 3C but the volume of a sub droplet is smaller. Sub droplets tend to have a smaller ejection speed as their volume decreases. Thus, the smaller the sub droplet, the larger the landing time difference between the sub droplet and the main droplet and therefore their distance. FIG. 3D shows a satellite formed separate from the main dot because of a larger landing time difference between the main droplet and the sub droplet than that of FIG. 3C.
As described above, the print position of satellite varies depending on various factors. When a bidirectional multi-pass printing is performed, dots formed in the forward scan and dots formed in the backward scan mix in the same image area (for example, the same pixel, the same pixel line or the same pixel area having M×N pixel).
FIG. 4 shows a variety of dot landing states when a bidirectional multi-pass printing is performed on a 2×2-pixel area. It is seen that the printed positions of satellites are inverted relative to the main dots depending on whether individual pixels are printed in the forward or backward main scan. In FIG. 4, a right-pointing arrow denotes a forward direction, a large circle with diagonal lines denotes a main dot printed by the carriage scanning in the forward direction, and a small circle with diagonal lines denotes a satellite printed by the carriage scanning in the forward direction. Furthermore a left-pointing arrow denotes a backward direction, a large white circle denotes a main dot printed by the carriage scanning in the backward direction, and a small white circle denotes a satellite printed by the carriage scanning in the backward direction.
As long as the satellites described above, if produced, are printed at the same position as the main dots or small enough compared with the main dots, no problem occurs in image quality. However, with a print head developed in recent years to eject very small ink droplets with high resolution, the main dots themselves have much smaller diameters and therefore the presence of satellites cannot be ignored. Particularly, when a secondary color is produced by overlapping two different inks, the problem becomes more serious.
FIGS. 5A to 5C show a case where cyan dots and magenta dots are overlapped to produce a blue color. As shown in the figure, two blue dots are formed in a 2×2-pixel area by moving the carriage in the direction of arrow. Here it is assumed that two print heads for cyan and magenta have the same satellite producing conditions. A satellite composed of two overlapping color dots is formed by the side of each blue dot formed of two main droplets. The satellites, formed by overlapping two different colors, are more conspicuous than when they are formed of a primary color, having greater effects on an image. If such distinctive satellites are produced unevenly, the uniformity is impaired, deteriorating the image quality.
To deal with the unevenness in landing position of satellites, some measures have already been proposed. For example, Japanese Patent Application Laid-open No. 2003-053962 discloses a technology that controls the feed distance of a print medium such that it includes at least an odd and even number of times the value of 1/D (D=printing resolution in the sub scan direction), in order to disperse the landing positions of satellites as possible and produce a uniform image.
With the method disclosed in the Japanese Patent Application Laid-open No. 2003-053962, however, a pixel in which satellites land on both sides of a main dots and a pixel in which satellites land insides of a main dots are arranged alternately. It is insufficiency for uniformity of image. Further, the method disclosed in the application provides a restriction on the control of transport distance of the print medium. Moreover, this technology does not take the secondary color described above into consideration, leaving the problem of easily noticeable secondary color satellites unsolved.